If you need to be sure the worms have been thoroughly dealt with, it's best to use Flubenvet. Other products which claim to prevent worms may well be good for the chicken's health as a suipplement, but will not be fully effective as a wormer. Measure out sufficient food for your hens for 8 days (they need a 7-day course but you don't want the feeder to go empty on the last day.) Take out a small amount of the food and add enough vegetable oil to coat the pellets, so the Flubenvet will stick to them and not just fall to the bottom of the feeder. Add the Flubenvet and mix very thoroughly. then gradually add the rest of the week's food, in batches, stirring carefully. Empty the feeders of untreated food, and fill with the treated pellets. Just give the pellets as food during the week's treatment, avoiding 'treats' which may prevent the hens eating enough of the pellets to be fuilly effective. No need for egg withdrawal whilst treating the birds.
I don't know why you should feel the need to worm such young chicks and poults as you mention. The very experienced breeder I bought some chicks from a few weeks ago told me not to worm them until they were at least 12 weeks, and if you've been keeping them in clean conditions I don't see why it shoul be necessary before about 18 weeks. I plan to worm all my flock when my growers are 18 weeks, before introducing them to the older birds.